So the reason for that is simple: the sensor gets heated as it is exposed to light, the more it heats itself, the more noise you get, so eventually after a one minute exposure, even i very low light, the image can be unbareably noisy. Also, as we know, the overheating of electronics is not anything welcome.
This is why full frame sensors suffer less from noise, as the neighbouring pixels are more apart, they influence one another to a lesser extent. Result - less noise at greater ISO. It works the other way round too, the small compact lenses are very dense with pixels, so they suffer from noise much more.
So if you're after really long exposures, you have to go for the film. Here's a video, explaining everything:
By the way, the tutorials they give are great not only for Nikon, they're good for understanding photography in general.